Cytochrome C oxidase is the terminal enzyme of cellular respiration in all higher aerobic organisms. It functions to catalyze the reduction of dioxygen to water, coupling the resultant energy through the cytochrome chain into the oxidation phosphorylation process. There is no enzyme system in any higher animal that is more important than cytochrome oxidase. Its metabolic role is essential to life. In spite of being one of the most studied enzyme systems in existence, it also remains one of the most enigmatic. By studying well chosen model compounds we can increase our understanding of this most important enzyme. The goals of this investigation are: a) to prepare Fe/Cu complexes which mimic the active site in cytochrome C oxidase, b) to examine the physical and chemical properties of these complexes, and c) to examine the mechanism of their reaction with dioxygen. Iron/copper mixed-metal complexes will be prepared and studied by the same series of methods and techniques that have been employed to study the natural enzyme. These will include: epr, electrochemistry, magnetic susceptibility, optical spectroscopy, Mossbauer spectroscopy, ligand binding studies, and simple reaction rate studies. In addition, X-ray diffraction studies will be carried out to establish the structures of these new mixed-metal complexes. The results of the model studies will be used in an effort to try to better understand the properties of the enzyme.